Imaging chips, such as CMOS sensors or CCDs, include circuitry which is capable of sensing images received by the chip. Imaging chips are typically encapsulated in a package that includes a lens of glass or some other type of transparent material that focuses an image onto the light sensitive circuitry of the chip. In response, the chip senses the actual image received by the imaging circuitry through the lens.
Imaging chips are usually affixed to a printed circuit board within some type of imaging system. Metal contacts between the chip and the board and metal traces on the board enable the imaging chip to communicate with other devices on the board.
A procedure known as “solder re-flow” is commonly used to affix most semiconductor devices onto printed circuit boards. During a standard solder re-flow operation, a solder paste is deposited onto contact pads of the chip package. Thereafter, the contacts are aligned and positioned onto mating contact pads of the printed circuit board. The package and board are then heated to temperatures sufficient to cause the solder paste to re-flow. When the heat is removed and the solder cools, bonds are formed at the contact points to affix the package to the board.
Solder re-flow operations are problematic with imaging chips. The high temperatures required to cause the solder to re-flow, typically 220 degrees C. or greater, may be harmful to the imaging circuitry on the chip. To overcome this problem, imaging chips are sometimes affixed to a printed circuit board using a laser to spot solder the contacts. With this procedure, a laser beam is focused specifically on the solder paste disposed between the contacts of the package and board. The heat from the laser causes the solder paste to re-flow. When the laser is removed, the solder cools, resulting in a bond between the package and the board. The advantage of spot welding is that the imaging circuitry is not exposed to elevated temperatures which helps prevent damage to the chip.
Spot welding of contacts, however, is less than ideal. Imaging chips tend to be highly integrated devices that require a high number of input/output contacts, typically arranged in an array located on the bottom surface of the chip package. It is therefore very difficult to spot solder the contacts located in the center region of the array away from the outer periphery of the chip package. Spot welding is also typically performed serially, one contact at a time. Thus the time involved to spot solder all the contacts of a chip is significant, particularly compared to a standard re-flow operation where all the contacts are re-flowed at the same time. The laser equipment is also expensive, which adds to the cost and complexity of printed circuit board assembly. Finally, once a chip is soldered to a printed circuit board, it is very difficult to remove without damaging the chip and/or board. It is therefore not feasible to either replace or upgrade in the field a chip package that has been soldered to a printed circuit board.
An apparatus and method for force mounting semiconductor packages onto printed circuit boards without the use of solder is therefore needed.